Actually, offensive linemen have become faster and more athletic over the years. Back then, the only athletes on the O-line were the guards. The center mostly blocked straight ahead on the nose tackle or MLB, and the tackles were slow-footed oafs who also blocked straight ahead.
Today, you have much more complex blocking schemes. Tackles and centers routinely pull and have to block OLBs, safeties and corners. You have guys like Gallery who weigh 325 and have 4.7 speed in the 40.
Roster weights are usually taken in training camp. I'll bet you Washington carries closer to 400 lbs by season's end. But under that blubber, there are quick muscles. I remember the Colts regular season game in 2003 where he beat his block and stopped Edgerinn James in the backfield on 4th and goal---saved the game for the Pats.
With guys like him and Sapp--it's not so much what they look like now, but what they did along the way to get to the point where they can carry all that weight without losing much quickness. Boil away the blubber and you'd see some impressive muscle mass underneath, I guarantee ya
On guys like Eller and Page, you saw big boned guys with those long, lean, natural muscles. Page, a judge these days, looks thin now but I'll bet you he still goes a good 225--all bone and lean muscle.